feat: add support for OpenAI-compatible third-party provider

[ChinaSiro]

Fixes #341

Key requirements:

• Accept OpenAI Chat/Completions request schema (model/messages/stream/etc.)
• Ensure compatibility with third-party providers using the OpenAI format (e.g., self-hosted gateways, multi-model routers)
• Support text, image, audio, video, and file inputs

[gemini-code-assist]

Summary of Changes

Hello @ChinaSiro, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a new openai package that provides an OpenAI-compatible model adapter. This adapter allows the system to seamlessly integrate with a wide range of third-party LLM providers that adhere to the OpenAI Chat/Completions API specification. The implementation includes robust request and response conversion, enabling support for advanced features like streaming, function calling, and multi-modal inputs, significantly expanding the system's flexibility and interoperability with the broader AI ecosystem.

Highlights

• OpenAI Compatibility: Introduces a new openai package that acts as an adapter for OpenAI-compatible APIs, allowing the model.LLM interface to interact with various third-party LLM providers.
• Comprehensive Feature Support: The adapter supports the OpenAI Chat/Completions request schema, including system instructions, streaming responses, and function calling capabilities.
• Multi-modal Input Handling: Extends capabilities to process diverse input types such as text, images, audio, video, and PDF files by converting genai.Blob data into appropriate OpenAI content formats.
• Detailed Example: A new main.go example is added to showcase the usage of the OpenAI-compatible model, demonstrating all supported features and multi-modal inputs.

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[gemini-code-assist]

Code Review

This pull request introduces a well-implemented and robust adapter for OpenAI-compatible APIs. The code is comprehensive, covering various features like streaming, function calling, and multimodal inputs. The inclusion of compatibility workarounds for non-standard third-party providers is a thoughtful addition. The unit tests are thorough, and the example usage is clear and helpful. I have one suggestion to improve performance in the content conversion logic.